Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is conventionally transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due in part to the large volume occupied by a gas. Because the volume of a gas is so much greater than the volume of a liquid containing the same number of gas molecules, the process of transporting natural gas over very long distances typically includes chilling and/or pressurizing the natural gas at or near the natural gas well in order to liquefy it. However, this contributes to the final cost of the natural gas.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline and middle distillates have been decreasing and supplies are not expected to meet demand in the coming years. Middle distillates typically include heating oil, jet fuel, diesel fuel, and kerosene. Fuels that are liquid under standard atmospheric conditions have the advantage that in addition to their value, they can be transported more easily in a pipeline than natural gas, since they do not require energy, equipment, and expense required for natural gas liquefaction.
Thus, for all of the above-described reasons, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted with oxygen to form syngas, which is a combination of carbon monoxide gas and hydrogen gas. In the second transformation, known as the Fischer-Tropsch process, carbon monoxide is reacted with hydrogen to form organic molecules containing carbon and hydrogen. Those organic molecules containing only carbon and hydrogen are known as hydrocarbons. In addition, other organic molecules containing oxygen in addition to carbon and hydrogen known as oxygenates may be formed during the Fischer-Tropsch process. Hydrocarbons having carbons linked in a straight chain are known as aliphatic hydrocarbons that may include paraffins and/or olefins. Paraffins are particularly desirable as the basis of synthetic diesel fuel.
Typically the Fischer-Tropsch product stream contains hydrocarbons having a range of numbers of carbon atoms, and thus having a range of molecular weights. Thus, the Fischer-Tropsch products produced by conversion of natural gas commonly contain a range of hydrocarbons including gases, liquids and waxes. Depending on the molecular weight product distribution, different Fischer-Tropsch product mixtures are ideally suited to different uses. For example, Fischer-Tropsch product mixtures containing liquids may be processed to yield gasoline, as well as heavier middle distillates. Hydrocarbon waxes may be subjected to an additional processing step for conversion to liquid and/or gaseous hydrocarbons. Thus, in the production of a Fischer-Tropsch product stream for processing to a fuel it is desirable to maximize the production of high value liquid hydrocarbons, such as hydrocarbons with at least 5 carbon atoms per hydrocarbon molecule (C5+ hydrocarbons).
The Fischer-Tropsch process is commonly facilitated by a catalyst. Catalysts desirably have the function of increasing the rate of a reaction without being consumed by the reaction. A feed containing carbon monoxide and hydrogen is typically contacted with a catalyst in a reaction zone that may include one or more reactors.
Common reactors include packed bed (also termed fixed bed) reactors, fluidized bed reactors and slurry bed reactors. Originally, the Fischer-Tropsch synthesis was carried out in packed bed reactors. These reactors have several drawbacks, such as temperature control, that can be overcome by gas-agitated slurry reactors or slurry bubble column reactors. Gas-agitated multiphase reactors sometimes called “slurry reactors” or “slurry bubble columns,” operate by suspending catalytic particles in liquid and feeding gas reactants into the bottom of the reactor through a gas distributor, which produces gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are typically converted to gaseous and liquid products. The gaseous products formed enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered from the suspending liquid by using different techniques like filtration, settling, hydrocyclones, magnetic techniques, etc. Gas-agitated multiphase reactors or slurry bubble column reactors (SBCRs) inherently have very high heat transfer rates, and therefore, reduced reactor cost. This, and the ability to remove and add catalyst online are some of the principal advantages of such reactors as applied to the exothermic Fischer-Tropsch synthesis. Sie and Krishna (Applied Catalysis A: General 1999, 186, p. 55), incorporated herein by reference in its entirety, give a history of the development of various Fischer Tropsch reactors.
Typically, in the Fischer-Tropsch synthesis, the distribution of weights that is observed such as for C5+ hydrocarbons, can be described by likening the Fischer-Tropsch reaction to a polymerization reaction with an Anderson-Shultz-Flory chain growth probability (a) that is independent of the number of carbon atoms in the lengthening molecule. α is typically interpreted as the ratio of the mole fraction of Cn+1 product to the mole fraction of Cn product. A value of α of at least 0.72 is preferred for producing high carbon-length hydrocarbons, such as those of diesel fractions.
The composition of a catalyst influences the relative amounts of hydrocarbons obtained from a Fischer-Tropsch catalytic process. Common catalysts for use in the Fischer-Tropsch process contain at least one metal from Groups 8, 9, or 10 of the Periodic Table (in the new IUPAC notation, which is used throughout the present specification).
Cobalt metal is particularly desirable in catalysts used in converting natural gas to heavy hydrocarbons suitable for the production of diesel fuel. Alternatively, iron, nickel, and ruthenium have been used in Fischer-Tropsch catalysts. Nickel catalysts favor termination and are useful for aiding the selective production of methane from syngas. Iron has the advantage of being readily available and relatively inexpensive but the disadvantage of a high water-gas shift activity by which carbon monoxide, instead of reacting with hydrogen to produce hydrocarbons, is rejected from the system as the undesired carbon dioxide while forming, rather than consuming, hydrogen (from the water). Ruthenium has the advantage of high activity but is quite expensive and insufficiently abundant.
Many petroleum and chemical processes use particularized catalysts for the conversion of a feedstock to one or more desired products, such as the Fischer-Tropsch process described herein. In any reaction requiring a catalyst, the catalyst can be expected to have a certain life, for example several months to a few years. Accordingly, as the time on stream increases the catalyst tends to degrade, and eventually becomes ineffective. The spent catalyst or a portion of the spent catalyst can be removed from a reactor vessel, and in order to maintain catalyst inventory into the reactor, new and/or regenerated catalyst can be loaded therein. The selection depends largely on the cost of manufacture of the catalyst and the ability for the catalyst activity to be restored. The removed spent catalyst can undergo a regeneration process if the activity of the spent catalyst removed from the reactor vessel can be at least partially restored. However, in some cases the loss of catalyst activity is irreversible and the spent catalyst can undergo a reclamation process to recover the valuable materials. While such reclamation processes may be located on site, they are often off site such that the spent catalyst must be transported for processing. In such cases, it is preferable to remove any residual hydrocarbonaceous products from the spent catalyst prior to processing the spent catalyst in order to recover the value of the hydrocarbonaceous products, avoid the additional transportation costs associated with the weight of the residual hydrocarbons, and minimize the presence of hydrocarbonaceous compounds in any waste materials for environmental conservation reasons. Therefore, a need exists in the art for efficient methods and systems for the removal of residual hydrocarbons from catalysts, and in particular Fischer-Tropsch catalysts that are used in large quantities, to facilitate the reclamation of such catalysts.